Wireless communication systems are widely deployed to provide various types of communication such as voice and data. These systems may be based on code division multiple access (CDMA), time division multiple access (TDMA), or some other modulation techniques. A CDMA system provides certain advantages over other types of systems, including increased system capacity.
A CDMA system may be designed to support one or more CDMA standards such as (1) the “TIA/EIA-95-B Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System” (the IS-95 standard), (2) the standard offered by a consortium named “3rd Generation Partnership Project” (3GPP) and embodied in a set of documents including Document Nos. 3G TS 25.211, 3G TS 25.212, 3G TS 25.213, and 3G TS 25.214 (the W-CDMA standard), (3) the standard offered by a consortium named “3rd Generation Partnership Project 2” (3GPP2) and embodied in a set of documents including “C.S0002-A Physical Layer Standard for cdma2000 Spread Spectrum Systems,” the “C.S0005-A Upper Layer (Layer 3) Signaling Standard for cdma2000 Spread Spectrum Systems,” and the “C.S0024 cdma2000 High Rate Packet Data Air Interface Specification” (the cdma2000 standard), and (4) some other standards.
One technique used to enhance performance, including system capacity and data throughput, is to lower the required transmit signal power by employing transmit diversity. Transmit diversity involves transmitting data on two or more antennas, where the geographical separation between the antennas leads to path loss characteristics that are independent from antenna to antenna. Thus, a receiving station can coherently combine signals from the transmit diversity antennas, and the noise introduced in the channel will not combine coherently, thus increasing the signal-to-noise ratio (SNR) received.
To maximize received energy and minimize errors, time tracking is performed to minimize phase differences between the pilot signals used for demodulation and the signals being demodulated. Techniques for time tracking in a transmit diversity system include methods for tracking signals from the plurality of antennas independently, as well as methods for tracking a weighted average of the signals from the various antennas with a single tracking loop. These methods are described in co-pending U.S. patent application Ser. No. 09/964,589, entitled “TIME TRACKING IN A NON-NEGLIGIBLE MULTIPATH SPACING ENVIRONMENT”, filed Sep. 25, 2001, and assigned to the assignee of the present invention.
Time tracking can be performed in discrete special purpose hardware, in a digital signal processor (DSP), or other general-purpose processor, or a combination of the two. Complexity in hardware introduces increased costs, often in terms of circuit area and power consumption. Additional complexity in software takes up processing time, perhaps leading to increased power consumption, or the requirement of more powerful and costly processors, or both. Simplified time tracking procedures that perform comparably with more complex procedures are clearly desirable. There is therefore a need in the art for an improved time tracking loop for diversity pilots.